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crop rotation-crop residue removal > monoculture-crop residue removal. According to the results, it can be concluded that soil organic matter is the central index of soil quality, which is intensely influenced by crop management system. Therefore, in arid and semiarid areas such as Iran, monitoring of the long-time effect of agronomic management on status of soil organic matter and soil physical indices is urgent.]]>
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2-5 mm). Although, there is not a unique acceptable methodology that serves and applies the entire world up to now, aggregate stability has been introduced as a soil physical quality indicator. Difficulties remain when comparison of aggregate stability from different methodologies are done. The objective of the present study was to assess appropriate and satisfactory aggregate stability tests that enable to distinguish the soil physical quality condition of both arid and moist medium textured soils.
Materials and Methods: A total of 120 soil samples which contained 60 wetland samples from Guilan province with a very humid climate, average annual rainfall of 1285 mm, and average annual temperature of 16°C, and 60 samples from Fars province with dry climate, average rainfall of 225 mm, and the average annual temperature of 27°C were provided. Soil sampling was performed from surface layer (0-20 cm). Each 10 soil samples with similar texture were mixed and one soil sample for each texture was finally obtained. After air drying and sieving, soil texture and organic carbon were determined by pipette and oxidation methods, respectively. Also, undisturbed samples were taken using metal cylinders from surface layer of 5-15 cm for determination of soil saturation coefficient, soil moisture curve, and soil bulk density. Also, in order to determine the aggregate stability, Kemper and Rosenau, de Leenheer and de Boodt, as well as Le Bissonnais were used.
Results and Discussion: Among different tested methods, wet sieving using the well known fast wetting methods of Kemper & Rosenau and of Le Bissonnais presented similar results in both climates. The mean weight diameter value of both methods for assessing aggregate stability can be considered as a dependable indicator of soil structure status for comparing soils. These aggregate stability tests were in correspondence with only one out of the eight soil physical quality indicators when the entire soils were used. It was concluded that the aggregate stability should be used judiciously and in accordance with other indicators for an overall assessing of the soil physical quality condition. The great differences in the estimation of aggregate stability between KRSW and LB2 with other methods confirm that aggregate stability increases with increasing soil moisture content. This involves reducing the amount of air condensed, which results in the reduction of compressive forces on the aggregates during rapid wetting. But the lack of similarity between the KRSW and LB2 methods in terms of MWD suggests that the results of these two methods are not comparable to dry and wet soils. The difference in aggregate size distribution from all three treatments of LB method was higher in dry areas than wet areas. Only dry soils based on LB (LB1 and LB3 treatments based on MWD) (P <0.05) are comparable. In dry soils, the LB3 method is very efficient. This method involves the use of ethanol that protects the aggregate structure against dryness stresses. The lack of similarity between the MWD and other soil quality indicators describes the complexity of the soil structure, which is dependent on the location. It seems that SOC can be considered as an indicator with high correlation with the aggregate sustainability index of LB and KRFW methods, at least in the studied medium-textured soils.
Conclusion: Since only a soil quality index (SOC) had a similar trend to the sustainability index derived from these two methods (LB1 and KRFW), it can be concluded that aggregate stability should be judged and recognized correctly, along with other used soil physical indicators for a general assessment of the conditions. In case of arid land soils, efficiency of pre-wetted methods of Kemper and Rosenau and of Le Bissonnais as well as pre-wetted Le Bissonnais with mechanical slaking and shaking were similar. If a simple and rapid analysis of the structure status is needed, single tests such as fast wetted Kemper and Rosenau and Le Bissonnais can be used.]]>
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